Manufacturing method for preventing pole piece from cracking and pole piece

Abstract

The invention discloses a manufacturing method for preventing a pole piece from cracking and the pole piece, and the method comprises the following steps: preparing double-layer composite slurry with matched thermal expansion coefficient gradient, coating the double-layer composite slurry on the surface of a copper foil, and coating surface slurry on bottom slurry; the coated pole piece sequentially passes through a copper foil side infrared radiation heating area, a coating surface cold and hot air alternate drying area and an alternating magnetic field treatment area, bottom layer slurry is preferentially shrunk through infrared radiation, surface layer solvent gradient volatilization is controlled through cold and hot air alternation, and a magnetic field induces graphene to be directionally arranged so as to fill microcracks; performing pulse laser scanning on the dried pole piece, and softening a surface adhesive to realize in-situ self-repairing; coating strain distribution is monitored in real time, and drying parameters are dynamically adjusted. Compared with the prior art, the manufacturing method can effectively prevent cracks from appearing on the surface of the pole piece.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery electrode sheet preparation, and particularly relates to a manufacturing method and an electrode sheet for preventing cracks in the electrode sheet. Background Art

[0002] In the manufacturing process of lithium-ion battery negative electrode sheets, the drying process after coating is a key link determining the coating quality. As the energy density of power batteries increases to ≥300 Wh / kg and the thickness of the negative electrode coating increases to 100 - 150 μm, the problem of interfacial shear stress caused by the difference in thermal expansion coefficients between the copper foil and the graphite coating becomes increasingly prominent. According to statistics, the crack density of negative electrode sheets after drying under traditional processes is as high as 8% - 12%, and 80% of the cracks (as Figure 1 shown) originate from stress concentration at the copper foil - coating interface, directly leading to a decrease in the battery cycle life and deterioration of the fast charging performance.

[0003] Among them, the existing solutions and limitations for the problem of cracking in negative electrode sheets are as follows: 1) Thermal stress mismatch: The differential expansion of the copper foil and the graphite coating during drying generates an interfacial shear stress as high as 18 MPa, and traditional cooling and drying can only reduce the stress to 12 MPa, unable to completely eliminate cracking; 2) Dependence on additives: To alleviate cracking, the industry generally adds NMP (N-methylpyrrolidone) as a plasticizer, but the cost of NMP accounts for 15% - 20% of the total cost of the negative electrode sheet, and its toxicity requires an additional waste gas treatment system; 3) Microstructural defects: The porosity distribution of traditional single-layer slurries is uneven after drying, causing lithium metal precipitation during fast charging and resulting in capacity drop.

[0004] Therefore, the above methods have the following common defects: 1) Treating the symptoms rather than the root cause: Only improving from the perspective of slowing down the drying rate or enhancing the bonding force, without touching on the fundamental cause of the thermal expansion coefficient mismatch; 2) High process complexity: Multi-layer coating requires additional equipment (such as a dual-mode head coater), and the yield loss is 8% - 10%; 3) Environmental protection and cost pressure: Dependence on solvents such as NMP leads to a risk of exceeding the VOCs emission standard, and the environmental protection compliance cost ratio increases.

[0005] Therefore, there is an urgent need to develop a manufacturing method for preventing cracks in electrode sheets that neutralizes interfacial stress from the source of material design and combines dynamic process control. Summary of the Invention

[0006] The purpose of the present invention is to provide, in view of the deficiencies of the prior art, a manufacturing method for preventing cracks in electrode sheets, which can effectively prevent cracks from occurring on the surface of the electrode sheet coating.

[0007] To achieve the above purpose, the present invention provides the following technical solutions:

[0008] A manufacturing method for preventing cracks in electrode sheets, comprising the following steps:

[0009] S1. Prepare a double-layer composite slurry: including a bottom slurry and a surface slurry with different coefficients of thermal expansion. The bottom slurry is coated on the surface of the copper foil, and the surface slurry is coated above the bottom slurry; S2. Heterogeneous gradient drying: The coated electrode is successively passed through the following three drying zones,

[0010] The first zone: Infrared radiation heating is carried out from the back of the copper foil, and the temperature is 70~80°C;

[0011] The second zone: Hot air and cold air are alternately blown on the surface of the coating. The temperature of the hot air is 60~70°C, and the temperature of the cold air is 20~25°C;

[0012] The third zone: An alternating magnetic field is applied and the temperature is maintained at 50~55°C;

[0013] S3. In-situ self-healing treatment: Pulse laser scanning is carried out on the dried electrode to soften the surface binder and seal potential cracks; S4. Dynamic stress monitoring and feedback: During the manufacturing process, the strain distribution of the coating is monitored in real time. When it is detected that the local strain exceeds the set threshold, the alternating frequency of hot and cold air in the second zone, the magnetic field strength in the third zone or the laser scanning path is dynamically adjusted to prevent the generation of new cracks.

[0014] Furthermore, the bottom slurry contains graphite (D50 = 10~15 μm), carbon nanotubes (0.3~0.7 wt%), polyimide binder, and the solid content is 48%~52%; the surface slurry contains graphite (D50 = 5~10 μm), graphene (0.5~1.5wt%), SBR / CMC binder, and the solid content is 45%~50%.

[0015] Furthermore, the coefficients of thermal expansion of the bottom slurry and the surface slurry satisfy:

[0016] α1−α2=(0.6∼1.2)×(α 铜箔 −α2)

[0017] where α1 is the coefficient of thermal expansion of the bottom slurry, and the value range of α1 is (14~18)×10 -6 / °C, α2 is the coefficient of thermal expansion of the surface slurry, and the value range of α2 is (5~8)×10 -6 / °C, α 铜箔 is 17×10 -6 / °C.

[0018] Furthermore, the alternating frequency of hot and cold air in the second zone is 0.2~0.5 Hz, the hot air velocity is 1~2 m / s, and the cold air velocity is 0.5~1 m / s.

[0019] Further, the intensity of the alternating magnetic field applied in the third region is 50 - 100 mT, and the frequency is 10 - 20 Hz.

[0020] Further, the wavelength of the pulsed laser is 1064 nm, the power density is 5 - 10 W / cm², the pulse width is 10 - 20 ns, and the scanning speed is 10 - 50 mm / s.

[0021] Further, the dynamic stress monitoring adopts the digital image correlation method (DIC), the spatial resolution ≤ 1 μm, and the sampling frequency ≥ 10 Hz.

[0022] Further, the set threshold is that the local strain ≥ 0.5%, triggering at least one of the following compensation operations:

[0023] a. Adjust the hot and cold air alternating frequency in the second region by ±0.1 Hz;

[0024] b. Increase the magnetic field intensity in the third region by ±10 mT;

[0025] c. Re - plan the laser scanning path to cover the strain - concentrated area.

[0026] Further, after the pulsed laser treatment, a porous structure with pore diameters of 0.5 - 5 μm is formed on the coating surface, and the electrolyte contact angle ≤ 10°.

[0027] Further, it also includes plasma cleaning of the copper foil before coating, using argon plasma, with a power of 100 - 200 W and a treatment time of 10 - 30 s.

[0028] Further, the wavelength of the infrared radiation in the first region is 3 - 5 μm, the radiation power density is 2 - 4 W / cm², and the heating time is 10 - 15 s.

[0029] Further, after the in - situ self - repair treatment, the interfacial resistance of the electrode sheet ≤ 2.5 Ω·cm², and the capacity retention rate after 1500 cycles ≥ 95%.

[0030] In addition, the present invention also provides an electrode sheet prepared by the above - mentioned method for preventing cracks in the electrode sheet.

[0031] Compared with the prior art, the present invention has at least the following beneficial effects:

[0032] 1) The present invention adopts the design of double - layer composite slurry (S1). By matching the thermal expansion coefficients of the bottom slurry and the surface slurry, the difference in thermal expansion coefficients between the copper foil and the coating is reduced, the interfacial shear stress is lowered, and the cracking of the coating is prevented.

[0033] 2) The present invention adopts heterogeneous gradient drying (S2). The infrared radiation in the first zone heats from the back of the copper foil, with a uniform temperature rise to reduce the temperature difference stress. In the second zone, hot and cold air blows alternately to control the drying rate of the coating surface and prevent stress concentration caused by too fast surface drying. In the third zone, an alternating magnetic field is applied to promote the orientation of the conductive additive, improve the conductivity and mechanical properties of the coating, and relieve stress at the same time.

[0034] 3) The present invention adopts in-situ self-healing treatment (S3). Pulse laser scanning softens the surface binder, fills the microcracks, and forms a dense coating. The porous structure formed by laser treatment improves the liquid affinity of the coating, which is beneficial to the infiltration of the electrolyte.

[0035] 4) The present invention adopts dynamic stress monitoring and feedback (S4). Through digital monitoring and feedback control, stress concentration areas can be detected and eliminated in time during the drying and treatment of the coating, preventing crack generation. The dynamic stress monitoring and feedback mechanism ensures the quality stability of each batch of products and reduces the defective rate.

[0036] In summary, the present invention adopts a double-layer composite slurry (bottom layer and surface layer) with different coefficients of thermal expansion. By reasonably matching the coefficients of thermal expansion, the thermal stress generated at the interface between the copper foil and the coating is reduced, and the cracking risk is significantly reduced. The introduction of heterogeneous gradient drying, in-situ self-healing treatment, and stress monitoring and feedback mechanism ensures uniform stress distribution in the coating during drying and subsequent treatment, thus avoiding the generation and expansion of coating cracks. By optimizing the raw materials and process parameters, the coating of the negative electrode sheet prepared by the present invention not only has a significantly reduced cracking rate, but also has better cycle life, fast charging performance, and interface stability. Description of the Drawings

[0037] Figure 1 It is a schematic diagram of a partial structure where cracks occur in the coating of the electrode sheet. Detailed Embodiments

[0038] Next, specific embodiments will be combined to clearly and completely describe the technical solutions of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0039] In the first aspect according to the present application, the present application provides a manufacturing method for preventing cracks in an electrode sheet, including the following steps:

[0040] S1. Prepare a double-layer composite slurry: including a bottom slurry and a surface slurry with different coefficients of thermal expansion. The bottom slurry is coated on the surface of the copper foil, and the surface slurry is coated above the bottom slurry; S2. Heterogeneous gradient drying: The coated electrode sheet is sequentially passed through the following three drying zones,

[0041] The first zone: Infrared radiation heating is carried out from the back of the copper foil, and the temperature is 70 - 80 °C;

[0042] The second zone: Hot air and cold air are alternately blown onto the surface of the coating. The temperature of the hot air is 60 - 70 °C, and the temperature of the cold air is 20 - 25 °C;

[0043] The third zone: An alternating magnetic field is applied and the temperature is maintained at 50 - 55 °C;

[0044] S3. In-situ self-healing treatment: The dried electrode is scanned by pulsed laser to soften the surface binder and seal potential cracks; S4. Dynamic stress monitoring and feedback: During the manufacturing process, the strain distribution of the coating is monitored in real time. When the detected local strain exceeds the set threshold, the alternating frequency of hot and cold air in the second zone, the magnetic field strength in the third zone, or the laser scanning path is dynamically adjusted to prevent the generation of new cracks.

[0045] In an embodiment according to the present application, the double-layer composite slurry adopts a continuous wet-on-wet coating process. After the bottom-layer slurry is coated, the surface-layer slurry is coated directly without drying; or after the bottom-layer slurry is pre-dried at 60 - 80 °C for 30 - 60 seconds, the surface-layer slurry is then coated. The continuous wet-on-wet coating process can ensure good bonding at the interface between the two layers of slurry and avoid a decrease in mechanical strength caused by interface delamination; the pre-drying process can avoid the mutual penetration and mixing of two different solvent systems (such as NMP and water) at the interface and improve interface stability.

[0046] In an embodiment according to the present application, the bottom-layer slurry contains graphite (D50 = 10 - 15 μm, 90 - 98 wt%), carbon nanotubes (0.5 - 2 wt%), polyimide binder (1.5 - 8 wt%), and the solid content is 48% - 52%; the carbon nanotubes and polyimide (PI) form a three-dimensional conductive network, and the copper foil - coating peel strength ≥ 15 N / cm; the bottom-layer solid content of 48% - 52% ensures the leveling property of the slurry, and the coating thickness deviation ≤ ±1 μm;

[0047] The surface-layer slurry contains graphite (D50 = 5 - 10 μm, 90 - 97 wt%), graphene (1 - 3.5 wt%), SBR / CMC binder (2 - 9 wt%), and the solid content is 45% - 50%; the graphene fills the gaps between graphite particles, the elastic modulus of the coating is increased, it is resistant to bending and has no cracks; the SBR / CMC binder forms an elastic buffer layer under the alternating hot and cold air, improving the stress release efficiency.

[0048] Therefore, the combination of the bottom slurry and the surface slurry makes the thermal expansion coefficient of the bottom layer close to that of the copper foil, while the thermal expansion coefficient of the surface slurry is lower. Through gradient design, the interfacial shear stress is effectively alleviated; the introduction of carbon nanotubes in the bottom layer improves the overall conductivity, while the addition of graphene in the surface layer enhances the mechanical strength and uniformity of the coating; the double-layer design makes the porosity distribution of the coating more uniform, effectively alleviating the problem of lithium metal precipitation during fast charging.

[0049] In an embodiment according to the present application, the thermal expansion coefficients of the bottom slurry and the surface slurry satisfy:

[0050] α1−α2=(0.6∼1.2)×(α 铜箔 −α2)

[0051] wherein, α1 is the thermal expansion coefficient of the bottom slurry, and the value range of α1 is (14~18)×10 -6 / ℃, α2 is the thermal expansion coefficient of the surface slurry, and the value range of α2 is (5~8)×10 -6 / ℃, α 铜箔 is 17×10 -6 / ℃.

[0052] Among them, strictly controlling the difference in the thermal expansion coefficients of the bottom layer and the surface layer to minimize the interfacial residual stress helps to reduce cracking caused by thermal stress; the theoretical design formula has been experimentally verified to ensure its applicability to the manufacture of electrode sheets under different process conditions and improve the yield rate.

[0053] In an embodiment according to the present application, the alternating frequency of hot and cold air in the second zone is 0.2~0.5 Hz, the hot air speed is 1~2 m / s, and the cold air speed is 0.5~1 m / s. Among them, the second zone is dried by alternating hot and cold air, and the thermal stress on the surface of the coating is further alleviated by cooling at a low speed. The hot air (60~70℃) promotes the volatilization of the surface solvent, and the cold air (20~25℃) induces shrinkage compensation, and the internal stress gradient ≤ 2 MPa / mm; when the alternating frequency is 0.2~0.5 Hz, the drying energy consumption is reduced. Preferably, the duration of the hot air accounts for 60-80% of a single cycle, and the cold air accounts for 20-40%.

[0054] In an embodiment according to the present application, the alternating magnetic field applied in the third zone is generated by an electromagnetic coil array, the magnetic field direction is perpendicular to the plane of the electrode sheet, the coil current waveform is a sine wave, the magnetic field intensity is 50~100 mT, the frequency is 10~20Hz, and the driving power is 200-500 W.

[0055] It should be noted that although carbon materials (graphite, carbon nanotubes, graphene) are usually weakly diamagnetic materials, there is certain electrical conductivity in graphene and carbon nanotubes. When in an alternating magnetic field, an induced current will be generated, and then an induced magnetic field will be produced. This induced magnetic field interacts with the applied magnetic field to generate a directional torque on the carbon material. At a magnetic field strength of 50 - 100 mT, this torque is sufficient to align carbon nanotubes and graphene in a specific direction.

[0056] Among them, the advantages of the alternating magnetic field (10 - 20 Hz) are as follows: Compared with the static magnetic field, the alternating magnetic field can generate periodic torque changes. This change helps conductive particles overcome the viscous resistance of the slurry and is more likely to be directionally arranged; at the same time, the periodic change of the magnetic field direction can make the conductive particles vibrate slightly, which is beneficial to releasing the internal stress during the drying process; in addition, the alternating magnetic field can also promote the volatilization of the solvent inside the coating and accelerate the drying process.

[0057] Therefore, the alternating magnetic field applied in the third region interacts with the magnetic anisotropy of the conductive carbon materials (carbon nanotubes, graphene) through the magnetic field direction, making the long axes of the particles arranged orderly along the magnetic field direction, thereby enhancing the continuity of the conductive network and reducing the interface resistance to ≤2.5 Ω·cm². When the frequency is 10 - 20 Hz, the microcrack initiation rate is reduced. This frequency range ensures that the arrangement rate is higher than the slurry curing rate, and at the same time avoids the Joule heating effect caused by high frequency.

[0058] In an embodiment according to the present application, the wavelength of the pulsed laser is 1064 nm, the power density is 5 - 10 W / cm², the pulse width is 10 - 20 ns, the scanning speed is 10 - 50 mm / s, and the laser scanning path is a spiral progressive type, covering the entire surface of the coating.

[0059] Among them, the ultrashort pulse width of 10 - 20 ns can achieve instantaneous heating in a microarea (200 - 300 °C), mainly based on the following mechanism: The laser of 1064 nm can be absorbed by specific functional groups (such as carbonyl, hydroxyl, etc.) in the SBR / CMC binder and instantaneously converted into heat energy; due to the extremely short pulse duration (10 - 20 ns), the heat is mainly concentrated in the surface layer within a thickness range of 5 - 10 μm and will not conduct to the deep layer. Therefore, only the surface layer of SBR / CMC is softened without damaging the graphite; this instantaneous high temperature is sufficient to briefly soften the surface binder, and it automatically fills the microcracks under the action of surface tension; after the pulse ends, the surface cools rapidly to form a stable repair layer.

[0060] The scanning speed of 10 - 50 mm / s can ensure that each position receives sufficient laser action, and at the same time, it will not cause overheating damage due to heat accumulation. Experimental verification shows that under the given laser parameters, the surface temperature of the coating can rapidly rise to 250 - 300 °C within 10 - 20 ns, which is sufficient to soften the SBR / CMC binder (softening temperature is about 150 - 200 °C), while graphite has high thermal stability (>1000 °C) and will not be damaged.

[0061] Laser treatment softens and makes the surface binder flow, thereby realizing in-situ repair of potential microcracks and significantly reducing the cracking rate; the microporous structure formed after laser treatment improves the wettability of the coating (electrolyte contact angle ≤ 10°), enhances the penetration of the electrolyte and the stability of the interfacial reaction, and helps to improve the fast charging performance; the laser action improves the arrangement of the internal particles of the coating and the contact performance of the interface, making the interfacial resistance of the electrode sheet ≤ 2.5 Ω·cm². Among them, the pore diameter after laser scanning is 0.5 - 5 μm, and the electrolyte infiltration time ≤ 10 s.

[0062] In an embodiment according to the present application, the dynamic stress monitoring adopts the digital image correlation method (DIC). The DIC system is arranged between the second zone and the third zone, and a high-speed camera (frame rate ≥ 100 fps) is used in cooperation with a random speckle pattern (sprayed to form, characteristic size 5 - 10 μm), the spatial resolution ≤ 1 μm, and the sampling frequency ≥ 10 Hz.

[0063] Among them, the working principle of the DIC system is as follows: A random speckle pattern is sprayed on the surface of the electrode sheet, and these speckles move as the electrode sheet deforms; the high-speed camera captures the position changes of these speckles in real time, and calculates the surface strain distribution through digital image processing algorithms; the system can complete the whole process from image acquisition to strain calculation within milliseconds, realizing real-time monitoring.

[0064] When implementing the DIC technology on the production line, the following method can be adopted: At the exit of the second zone, a small amount of inorganic pigment (such as titanium dioxide) is sprayed on the surface of the electrode sheet through a high-precision spraying device to form random speckles; the high-speed camera is installed between the second zone and the third zone, and the surface of the electrode sheet is photographed in real time through a transparent observation window; the collected images are analyzed by a high-speed processor to calculate the local strain distribution; when the detected strain exceeds the threshold, the system automatically issues an adjustment instruction to the corresponding process unit.

[0065] By using the DIC technology to monitor the coating stress distribution in real time, it is possible to intervene in time before cracking occurs, significantly reducing the crack density; dynamically adjusting the drying and laser repair process parameters to adapt to different coating thicknesses and material properties, improving the flexibility and adaptability of the process; through the dynamic feedback mechanism, it is possible to significantly reduce the product scrap caused by local strain concentration and improve the overall yield.

[0066] In an embodiment according to the present application, the set threshold is local strain ≥ 0.5%, triggering at least one of the following compensation operations:

[0067] a. Adjust the hot and cold air alternating frequency in the second zone by ±0.1 Hz;

[0068] b. Enhance the magnetic field strength in the third zone by ±10 mT;

[0069] c. Re-plan the laser scanning path to cover the strain concentration area.

[0070] Among them, through the compensation operation, real-time monitoring and dynamic adjustment of the coating stress are realized, preventing cracking caused by stress concentration, and improving the stability of the production process and product consistency.

[0071] When local strain ≥ 0.5% is detected, the system will preferentially execute the adjustment of the hot and cold air alternating frequency because this is the fastest response means; if the strain continues to increase, the adjustment of the magnetic field strength will be started simultaneously; for the area that has been dried but still has a large strain, the system will transmit its coordinate information to the laser scanning controller to strengthen the scanning intensity of this area in subsequent laser processing.

[0072] In an embodiment according to the present application, after the pulsed laser treatment, a porous structure with pore diameters of 0.5 - 5 μm is formed on the coating surface, and the electrolyte contact angle ≤ 10°. This microporous structure is the result of the instantaneous action of the laser and is mainly formed by the following mechanism: the laser energy causes local vaporization of the surface binder, leaving tiny holes; after the pulse ends, the surrounding materials quickly cool and solidify, maintaining the hole structure; these micropores significantly increase the surface area, improve the wettability of the electrolyte, reduce the electrolyte contact angle from 25 - 30° in the traditional process to ≤ 10°, and shorten the wetting time by more than 80%.

[0073] In an embodiment according to the present application, it also includes plasma cleaning of the copper foil before coating, using argon plasma, with a power of 100 - 200 W and a treatment time of 10 - 30 s. Among them, plasma cleaning effectively removes the oil stains and oxide layers on the copper foil surface, enhances the adhesion of the slurry; at the same time, it improves the interface bonding strength and further reduces the cracking risk. The action mechanism of plasma cleaning is: high-energy argon ions bombard the copper foil surface to physically remove pollutants; at the same time, it activates the surface, increases the surface energy, and improves the bonding force with the slurry; after treatment, the contact angle of the copper foil surface is reduced by more than 40%, and the peel strength is increased by 20 - 30%.

[0074] In an embodiment according to the present application, the wavelength of the infrared radiation in the first zone is 3 - 5 μm, the radiation power density is 2 - 4 W / cm², and the heating time is 10 - 15 s. This can precisely control the heating rate of the copper foil and avoid stress concentration caused by a sharp temperature gradient. The selection of infrared radiation with a wavelength of 3 - 5 μm is based on the absorption characteristics of the copper foil in this wavelength range, enabling efficient heat transfer; the method of heating from the back of the copper foil ensures uniform heat conduction from the current collector to the active material layer, avoiding the problem of overheating on the surface and underheating inside; controlling the heating time within 10 - 15 s achieves a mild and uniform heating process, with a temperature gradient ≤5℃ / mm, significantly lower than 15 - 20℃ / mm in traditional drying.

[0075] In an embodiment according to the present application, after the in-situ self-healing treatment, the interfacial resistance of the electrode sheet is ≤2.5 Ω·cm², and the capacity retention rate after 1500 cycles is ≥95%.

[0076] Among them, the main process mechanism analysis of the present application is as follows:

[0077] 1) Design principle of the double-layer composite slurry: The thermal expansion coefficient of the bottom slurry is close to that of the copper foil, reducing interfacial stress; the thermal expansion coefficient of the surface slurry is relatively low, providing overall stability; a thermal expansion coefficient gradient is formed between the two layers, avoiding sudden changes in stress. This gradient design is the core innovation of the present invention, solving the problem of thermal expansion coefficient mismatch from the source of material design.

[0078] 2) Action mechanism of heterogeneous gradient drying: The first zone heats from the back of the copper foil to ensure uniform heat conduction; the second zone is treated with alternating hot and cold air to control stress release during solvent evaporation; the third zone is treated with an alternating magnetic field to promote the directional arrangement of conductive particles and further release stress. This multi-stage gradient drying process avoids the problems of large temperature gradients and stress concentration in traditional drying.

[0079] 3) Microscopic mechanism of pulsed laser self-healing: The instantaneous high temperature generated by the pulsed laser softens and flows the surface binder, filling potential microcracks; the microporous structure formed by the laser action improves the wettability of the electrolyte; the entire process only affects the surface layer of 5 - 10μm and does not damage the internal active material. Laser treatment is an efficient surface modification method that can achieve precise defect repair.

[0080] 4) Control logic of dynamic stress monitoring and feedback: The DIC system monitors the surface strain distribution in real time. When it detects that the local strain exceeds the threshold, it automatically adjusts the process parameters; this closed-loop control ensures the stability of the production process and product consistency, significantly reducing the rejection rate.

[0081] In a second aspect according to the present application, the present application further provides a negative electrode sheet, comprising a copper foil substrate, a bottom slurry layer and a surface slurry layer, wherein the coefficient of thermal expansion of the bottom slurry layer is (14~18)×10 -6 / °C, the coefficient of thermal expansion of the surface slurry layer is (5~8)×10 -6 / °C, the crack density on the surface of the electrode sheet is ≤0.15%, the interfacial resistance is ≤2.5 Ω·cm², and it is obtained by the above-mentioned manufacturing method for preventing cracks in the electrode sheet.

[0082] This double-layer structure negative electrode sheet has the following characteristics: the difference in the coefficient of thermal expansion between the copper foil and the bottom slurry layer is small, significantly reducing the interfacial stress; a thermal expansion coefficient gradient is formed from the bottom layer to the surface layer, avoiding stress concentration between layers; the low coefficient of thermal expansion of the surface layer makes the overall structure more stable when the temperature changes; the overall crack density is controlled below 0.15%, far lower than 8-12% of the traditional process; the interfacial resistance is ≤2.5 Ω·cm², ensuring excellent electrochemical performance.

[0083] Next, the implementation and advantages of the present application will be further described in conjunction with specific embodiments.

[0084] Example 1

[0085] A manufacturing method for preventing cracks in a negative electrode sheet provided in this example includes the following steps:

[0086] Step S1: Prepare a double-layer composite slurry

[0087] 1) Preparation of the bottom slurry:

[0088] Graphite (D50 = 12 μm): 95 wt%;

[0089] Carbon nanotubes (1.5 wt%): diameter 10~20 nm, length 5~15 μm;

[0090] Polyimide (PI) binder: 3.5 wt%;

[0091] Solvent: NMP (residual amount ≤0.1%);

[0092] The solid content of the bottom slurry is 50%;

[0093] 2) Viscosity adjustment: After three-stage shearing (1000→2000→3000 rpm, total time 40 min) to 1800 mPa·s (25°C).

[0094] 3) Preparation of the surface slurry:

[0095] Graphite (D50 = 8 μm): 93wt%;

[0096] Graphene (2 wt%): The sheet thickness is 3 - 5 layers, and the lateral size is 5 - 10 μm;

[0097] SBR / CMC binder (mass ratio 3:1): 5 wt%;

[0098] Solvent: NMP (residual amount ≤ 0.1%);

[0099] The solid content of the surface slurry is 48%;

[0100] 4) Viscosity adjustment: Dispersed by a twin-screw extruder (rotation speed 500 rpm, time 30 min) to 1000 mPa·s (25 °C).

[0101] 5) Coating process: After the bottom slurry is coated, it is pre-dried at 70 °C for 45 seconds and then the surface slurry is coated.

[0102] 6) Among them, the thermal expansion coefficients of the bottom slurry and the surface slurry are: bottom layer α1 = 16×10 -6 / °C, surface layer α2 = 7×10 -6 / °C, and α1−α2 = 0.9×(α 铜箔 −α2).

[0103] Step S2: Heterogeneous gradient drying

[0104] 1) First zone (heating from the copper foil side):

[0105] The wavelength of the infrared radiation panel is 3.5 μm, and the power density is 3 W / cm²;

[0106] The heating time is 12 s, and the temperature on the back of the copper foil rises to 75 °C;

[0107] The solvent evaporation rate of the bottom slurry is 25%.

[0108] 2) Second zone (alternating hot and cold air):

[0109] Hot air: 65 °C, wind speed 1.5 m / s, time 10 s;

[0110] Cold air: 23 °C, wind speed 0.8 m / s, time 5 s;

[0111] The alternating frequency is 0.3 Hz (each cycle is 15 s), and the total processing time is 30 s;

[0112] The solvent evaporation rate of the surface slurry is 40%.

[0113] 3) Third zone (alternating magnetic field treatment):

[0114] The alternating magnetic field generated by the electromagnetic coil array, and the magnetic field direction is perpendicular to the plane of the electrode;

[0115] Magnetic field strength: 80 mT, frequency: 15 Hz, driving power: 350 W;

[0116] Temperature: 52 °C, time: 18 s;

[0117] Residual solvent content ≤ 500 ppm.

[0118] Step S3: In-situ self-healing treatment

[0119] Pulsed laser parameters: wavelength 1064 nm, power density 8 W / cm², pulse width 15 ns, scanning speed 30 mm / s;

[0120] The scanning path is a spiral progressive type, covering the entire surface of the coating;

[0121] The surface temperature instantaneously rises to 250 °C, and SBR softens and flows to fill potential microcracks.

[0122] Step S4: Dynamic stress monitoring and feedback

[0123] The DIC system (resolution 0.8 μm, sampling frequency 15 Hz) monitors the strain, and the threshold is set to 0.5%;

[0124] When the detected strain ≥ 0.6%, the alternating frequency of hot and cold air is automatically adjusted to 0.4 Hz, and the magnetic field strength is increased to 90 mT.

[0125] Example 2

[0126] Differing from Example 1, the relevant parameters of this example are as follows:

[0127] 1) The coating speed is increased to: 90 m / min;

[0128] 2) The drying time is compressed: 10 s (80 °C) in the first zone, 25 s (0.4 Hz) in the second zone, 15 s in the third zone;

[0129] 3) The laser scanning speed is increased to 50 mm / s.

[0130] The others are the same as in Example 1 and will not be elaborated here.

[0131] Example 3

[0132] Differing from Example 1, the relevant parameters of this example are as follows:

[0133] 1) The solvents of the bottom layer slurry and the surface layer slurry are replaced with water (adding 0.1 wt% dispersant);

[0134] 2) The solid content of the surface layer slurry is increased to 50%;

[0135] 3) The alternating magnetic field strength is increased to 100 mT, the frequency is adjusted to 20 Hz, and the driving power is 500 W.

[0136] The others are the same as in Example 1 and will not be elaborated here.

[0137] Comparative Example 1

[0138] Different from Example 1, this comparative example uses a traditional single-layer slurry with the following parameters:

[0139] Single-layer slurry: graphite (D50 = 10 μm), SBR / CMC, solid content 50%;

[0140] Drying process: constant temperature 70°C, wind speed 2 m / s, time 120 s.

[0141] The others are the same as in Example 1 and will not be elaborated here.

[0142] Comparative Example 2

[0143] Different from Example 1, this comparative example uses no alternating magnetic field, that is, the alternating magnetic field is cancelled in the third zone, and the drying time is extended to 70 s.

[0144] The others are the same as in Example 1 and will not be elaborated here.

[0145] Comparative Example 3

[0146] Different from Example 1, this comparative example uses no laser repair, that is, step S3 is omitted and it is directly cooled after drying.

[0147] The others are the same as in Example 1 and will not be elaborated here.

[0148] The following performance tests were respectively carried out on the pole pieces obtained in the above examples and comparative examples, and the test results are shown in Table 1.

[0149] 1) Interface shear stress test:

[0150] Equipment: micro-mechanical tester (Instron 5943), crosshead speed 0.1 mm / min;

[0151] Method: Peel the coating from the copper foil, record the maximum shear force, and calculate the stress = force / contact area.

[0152] 2) Crack density detection:

[0153] Equipment: SEM (Hitachi SU5000), observe 10 fields of view (1 mm × 1 mm), and count the total crack length / area.

[0154] 3) Cycle performance test:

[0155] Assemble a half-cell (Li metal counter electrode), electrolyte 1M LiPF6 (EC:DMC = 1:1), charge and discharge at 0.5C, voltage 0.01~2.0 V;

[0156] Capacity retention rate = 1500th discharge capacity / first discharge capacity × 100%.

[0157] 4) Interface resistance test:

[0158] Equipment: EIS (BioLogic VMP-3), frequency 100 kHz~0.01 Hz, amplitude 10 mV;

[0159] Obtain the interface resistance (Rint) by fitting the equivalent circuit.

[0160] Table 1 Group Interface shear stress (MPa) Crack density (%) Drying time (s) Cycle capacity retention rate (%) Interface resistance (Ω·cm²) Example 1 4.8 0.08 60 96.2 1.6 Example 2 5.3 0.12 50 94.7 2.0 Example 3 6.0 0.15 55 92.8 2.5 Comparative Example 1 17.8 11.5 120 83.2 5.6 Comparative Example 2 6.2 0.9 70 91.5 3.8 Comparative Example 3 5.1 0.7 60 89.3 4.2

[0161] The analysis of the above test results is as follows:

[0162] 1. Comparative analysis between the examples and the comparative examples:

[0163] 1) Interface shear stress: The interface shear stress of Example 1 is significantly lower than that of Comparative Example 1, proving the core role of the double-layer slurry thermal expansion gradient design; in Comparative Example 2, due to the absence of an alternating magnetic field, the stress release is insufficient, but it is still better than the traditional process. This stress reduction effect is mainly attributed to the close proximity of the thermal expansion coefficient of the bottom slurry (16×10 -6 / ℃) to that of the copper foil (17×10 -6 / ℃), which reduces the interface stress concentration.

[0164] 2) Crack density: The crack density of Example 1 is significantly lower than that of Comparative Example 3, highlighting the necessity of laser self-repair; in Comparative Example 2, due to the absence of magnetic field regulation, the graphene arrangement is disordered, and the crack inhibition effect is limited.

[0165] 3) Cycling performance: The capacity retention rate of Example 1 is 96.2%, significantly higher than that of Comparative Example 1, due to the low interface resistance and self-repair structure; in Comparative Example 3, due to the lack of laser repair, microcracks lead to the shedding of active materials and accelerated capacity decay.

[0166] 4) Interface resistance: The interface resistance of Example 1 is much lower than that of Comparative Example 1. This low interface impedance is beneficial to improving the rate performance of the battery and reducing internal heat generation. The synergistic effect of alternating magnetic field treatment and laser repair significantly improves the continuity of the conductive network and the interface contact quality.

[0167] 2. Comparative analysis between the examples

[0168] 1) Compared with Example 1, the drying time in Example 2 was compressed to 50 s. Although the crack density increased slightly, it was still far lower than that of the traditional process, indicating that good crack suppression effect could still be maintained in the high-speed mode; the interface resistance increased slightly, probably because the increase in laser scanning speed led to a decrease in pore uniformity, but the overall performance was still excellent.

[0169] 2) Compared with Example 1, Example 3 adopted a NMP-free formulation and completely used water as the solvent, meeting the environmental protection requirements. Although the interface resistance increased to 2.5 Ω·cm², the environmental protection performance was significantly improved, and the crack density still remained at a low level of 0.15%, far lower than that of the traditional process. This shows that the process of the present invention has good adaptability and can be applied to different solvent systems, providing technical support for the manufacture of green and environmentally friendly batteries.

[0170] 3) The capacity retention rates of Examples 1 - 3 were 96.2%, 94.7% and 92.8% respectively, all significantly higher than those of the comparative examples, indicating the stability and reliability of the process of the present invention. Even under the conditions of environmental protection formulation (Example 3) and high-speed production (Example 2), the electrochemical performance still remained at a relatively high level.

[0171] Based on the disclosure and teachings of the above specification, those skilled in the art to which the present invention pertains can also make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments, and any obvious improvements, substitutions or variations made by those skilled in the art on the basis of the present invention all fall within the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A method for preventing cracks in a pole piece, characterized in that: The following steps are involved: S1. Preparation of double-layer composite slurry: including a bottom slurry and a surface slurry with different thermal expansion coefficients, the bottom slurry is coated on the surface of the copper foil, and the surface slurry is coated on top of the bottom slurry; S2. Heterogeneous gradient drying: the coated electrode is sequentially passed through the following three drying zones, Zone 1: Infrared radiation heating from the back of the copper foil, temperature 70~80℃; Second zone: Blow hot air and cold air alternately to the coating surface, with the hot air temperature at 60~70℃ and the cold air temperature at 20~25℃; Zone 3: Apply alternating magnetic field and maintain temperature at 50~55℃; S3. In-situ self-repair treatment: Pulse laser scanning is performed on the dried electrode to soften the surface adhesive and seal potential cracks. S4. Dynamic stress monitoring and feedback: Real-time monitoring of coating strain distribution during the production process. When it is detected that the local strain exceeds the set threshold, the alternating frequency of hot and cold air in the second zone, the magnetic field strength in the third zone or the laser scanning path are dynamically adjusted to prevent new cracks from occurring.

2. The method for preventing cracks in a pole piece according to claim 1, characterized in that: The bottom slurry contains graphite, carbon nanotubes, and polyimide binder, and the solid content is 48% to 52%; the surface slurry contains graphite, graphene, and SBR / CMC binder, and the solid content is 45% to 50%.

3. The method for preventing cracks in a pole piece according to claim 1, characterized in that: The thermal expansion coefficients of the bottom slurry and the surface slurry satisfy: α1−α2=(0.6∼1.2)×(α 铜箔 −α2) Among them, α1 is the thermal expansion coefficient of the bottom slurry, and the value range of α1 is (14~18)×10 -6 / ℃, α2 is the thermal expansion coefficient of the surface slurry, and the value range of α2 is (5~8)×10 -6 / ℃,α 铜箔 17×10 -6 / ℃.

4. The method for preventing cracks in a pole piece according to claim 1, characterized in that: The hot and cold air alternating frequency of the second zone is 0.2~0.5 Hz, the hot air speed is 1~2 m / s, and the cold air speed is 0.5~1 m / s.

5. The method for preventing cracks in a pole piece according to claim 1, characterized in that: The alternating magnetic field applied in the third zone has an intensity of 50-100 mT and a frequency of 10-20 Hz.

6. The method for preventing cracks in a pole piece according to claim 1, characterized in that: The wavelength of the pulse laser is 1064 nm, the power density is 5-10 W / cm², the pulse width is 10-20 ns, and the scanning speed is 10-50 mm / s.

7. The method for preventing cracks in a pole piece according to claim 1, characterized in that: The set threshold is local strain ≥ 0.5%, triggering at least one of the following compensation operations: a. Adjust the hot and cold air alternation frequency of the second zone by ±0.1 Hz; b. Increase the magnetic field strength in the third zone by ±10 mT; c. Replan the laser scanning path to cover the strain concentration area.

8. The method for preventing cracks in a pole piece according to claim 1, characterized in that: It also includes plasma cleaning of the copper foil before coating, using argon plasma with a power of 100~200 W and a processing time of 10~30 s.

9. The method for preventing cracks in a pole piece according to claim 1, characterized in that: The wavelength of the infrared radiation in the first zone is 3~5 μm, the radiation power density is 2~4 W / cm², and the heating time is 10~15 s.

10. A pole piece, characterized in that: The method for preventing cracks in pole pieces is produced by any one of claims 1 to 9.

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